1. Field of the Invention
The present invention pertains to an image compression method for compressing digital image data, such as a full-color image with additive primary colors (RGB) or subtractive primary colors (CMY) or a monochrome image with 256 levels (gray scale).
2. Description of the Related Art
In general, digital image data comprising luminance information for the respective color elements, that is, respective color channels (color model), in a color model, such as the RGB system or the CMY system, is utilized for computer processing of a color image. Said digital image data is represented by 1 byte (8 bits) of information for expressing the luminance levels of the respective colors (RGB) of 1 pixel using values in 256 steps (256 gradations), that is, 0 through 255, so that 1 pixel has 3 bytes of information. Thus, a superfine color image contains an enormous amount of data, which means that not only hardware resources, such as a large-capacity memory, a hard disk drive, a high-speed CPU, and communications technology capable of realizing high-speed data transfer, but also a data compression technique are required in order to process said image data using a computer.
Although a variety of data compression methods have been suggested, some of these use techniques that reduce the quality when a compressed image is decompressed. For example, based on the fact that image data does not necessarily use every color, there is a method in which appropriate, preset color subtraction is applied in order to reduce the file size without destroying the original image. With this kind of compression method involving color subtraction, because similar color values are integrated during the compression process, the continuity of the color scale is lost, and fine lines, in particular, tend to become blurred. In addition, there is a known compression method in which luminance information is converted into color difference information when the color information is stored. However, the amount of significant information contained in color difference information is approximately ¼ or less that in luminance information, which means that hues tend to be reproduced incorrectly.
Japanese Kokai Patent Application No. Hei 10[1998]-32719 discloses an image compression method and a device in which color image data comprising luminance information for the 3 primary colors is converted into color model data comprising 1 luminance information and 2 color difference information which is then compressed using a method capable of high quality reproduction in order to achieve a sufficiently high compression rate and reproduced image quality. 3D vector quantization is adopted for said compression processing technique for high-quality reproduction; wherein, a group of horizontal pixel groups having luminance levels which can be assumed to change successively within a fixed allowable error range on an XZ plane, is expressed using a single vector unit, where distribution of the luminance levels of respective pixels on the XY image plane is expressed in terms of magnitude in the direction of the Z axis, and a group of vertical pixel groups having luminance levels which can be assumed to change successively within a fixed allowable error range on the XZ plane is expressed using a single vector unit in the same manner. When the image compression method according to the aforementioned Kokai patent application is utilized, while the compression rate can be improved significantly over the prior art, because conversion into color information is applied before compression, the reproduced picture quality tends to be degraded when the image data is expanded by decompression.
Thus, the purpose of the present invention is to present an image compression method by which successive changes in hue can be reproduced accurately, fine lines can also be reproduced clearly, and a high-quality image good enough to serve as an image manuscript for printing can be reproduced while also realizing a superior compression effect with superfine digital image data.